Vertical shear weld wafer bonding

ABSTRACT

In described examples, a first metal layer is configured along a periphery of a cavity to be formed between a first substrate and a second substrate. A second metal layer is adjacent the first metal layer. The second metal layer includes a cantilever. The cantilever is configured to deform by bonding the first substrate to the second substrate. The deformed cantilevered is configured to impede contaminants against contacting an element within the cavity.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/215,628 filed Dec. 10, 2018, which is incorporated herein byreference.

BACKGROUND

Microelectromechanical system (MEMS) devices such as actuators,switches, motors, sensors, variable capacitors, spatial light modulators(SLMs) and similar microelectronic devices can be manufactured on asubstrate. To protect such devices, sidewalls are formed on thesubstrate during manufacturing to form a sealable cavity, such thatstructures and devices within the cavity can be relatively isolated froman outside environment. However, contaminants can gradually migrate intothe cavity, and can react with or otherwise interfere with properoperation of devices included within the cavity.

SUMMARY

In described examples, a first metal layer is configured along aperiphery of a cavity to be formed between a first substrate and asecond substrate. A second metal layer is adjacent the first metallayer. The second metal layer includes a cantilever. The cantilever isconfigured to deform by bonding the first substrate to the secondsubstrate. The deformed cantilevered is configured to impedecontaminants against contacting an element within the cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of an example first-substrateassembly for hermetic vertical shear weld wafer bonding.

FIG. 2 is a cross-sectional diagram of an example first-substrateassembly including a first metal for hermetic vertical shear weld waferbonding.

FIG. 3 is a cross-sectional diagram of an example first-substrateassembly including a second metal for hermetic vertical shear weld waferbonding.

FIG. 4 is a cross-sectional diagram of an example first-substrateassembly including an exposed vertical edge of a fulcrum for hermeticvertical shear weld wafer bonding.

FIG. 5 is a cross-sectional diagram of example first- andsecond-substrate assemblies for hermetic vertical shear weld waferbonding.

FIG. 6 is a cross-sectional diagram of example joined first- andsecond-substrate assemblies for hermetic vertical shear weld waferbonding.

FIG. 7 is a cross-sectional elevational view of an example two-substrateassembly that includes example hermetic vertical shear weld wafer bonds.

FIG. 8 is a top-view diagram of an example first-substrate assembly forhermetic vertical shear weld wafer bonding.

FIG. 9 is a top-view diagram of an example first-substrate wafer forhermetic vertical shear weld wafer bonding.

FIG. 10 is a cross-sectional diagram of dimensioning of components ofexample first- and second-substrate assemblies for hermetic verticalshear weld wafer bonding.

FIG. 11 is another cross-sectional diagram of example first- andsecond-substrate assemblies for hermetic vertical shear weld waferbonding.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In this description: (a) the term “portion” can mean an entire portionor a portion that is less than the entire portion; (b) the term “formedon a substrate” can mean being arranged such that the “formed” object issupported by the substrate and extends above a preexisting surface ofthe substrate; (c) the terms “inwards” and “inner” can refer to adirection towards a cavity formed on a substrate; (d) the terms“outwards” and “outer” can refer to a direction away from a cavityformed on a substrate, such as a direction towards a wafer edge, a dieedge, or a saw lane; (e) the terms “downwards” and “lower” can refer toa direction towards a first substrate, such as a silicon substrate; and(f) the terms “upwards” and “upper” can refer to a second substrate,such as a glass wafer.

Microelectromechanical system (MEMS) devices such as actuators,switches, motors, sensors, variable capacitors, spatial light modulators(SLMs) and similar microelectronic devices can have movable elements.For example, an SLM device can include an array of movable elements.Each such element can be an individually addressable light modulatorelement in which an “on” or “off” position is set in response to inputdata. The input data can be image information to command light modulatorelements of the array to either project or block light directed at thearray from an illumination source.

In an example SLM device of an image projection system, the input dataincludes bit frames generated in response to pixel hue and intensityinformation data of an image frame of an image input signal. The bitframes can be projected using a pulse-width modulation. Pulse-widthmodulation schemes include weighted time intervals for projection ofpixels of pixel hue and intensity corresponding to respective pixels inthe input data. The weighted time intervals are sufficiently long topermit human eye integration over a given image frame display period. Anexample of an SLM device is a digital micromirror device (DMD), such asa Texas Instruments DLP® micromirror array device. Such DMD devices havebeen commercially employed in a wide variety of devices, such astelevisions, cinemagraphic projection systems, business-relatedprojectors and picoprojectors.

DMD devices can be manufactured to include micromirrors to digitallyimage and project an input digital image onto a display surface (such asa projection screen). For example, a projector system can include a DMDdevice can be included arranged to modulate an incident beam of lightreceived through a window glass of the DMD device and focused onmicromirrors therein. Each such micromirror can be individually anddynamically adjusted in response to input data to project a visual imageonto a projection screen.

An individual micromirror can be formed as a portion of a torsionspring. When the mirror goes “hard over,” the mirror contacts (e.g.,hits) a stopping surface. Occasionally, the contacting mirror encountersenvironmentally induced adhesion (e.g., stiction) forces sufficient toprevent the mirror from rebounding from the stopping surface. Suchstiction can result from environmental contamination and can createdefects and reliability problems.

Another such problem is excessive dynamic friction, which can resultfrom contact between moving elements in a MEMS device. Both theexcessive dynamic friction and the incidence of adhesion can be reducedby coating surfaces of the moving elements of a MEMS device with apassivating agent or lubricant (e.g., “lube”).

However, the passivating agents and lubricating coatings can becompromised by other chemical species used to manufacture a MEMS device.Over time, chemical species can migrate and then degrade the performanceof moving elements of a MEMS device. Such coatings for MEMS devices aredescribed in U.S. patent application Ser. No. 14/333,829, filed Jul. 17,2014, entitled “Coatings for Relatively Movable Surfaces,” by W.Morrison, et al., which is incorporated herein by reference in itsentirety for all purposes.

In the manufacture of semiconductors and MEMS devices, each MEMS deviceis manufactured using wafer-scale processing techniques. For example, awafer can include many like MEMS devices arranged in rows and columns(e.g., in an array) on a substrate of a single wafer. Such techniquescan decrease costs because many devices can be processed in parallel bysimultaneously applying process steps. Various MEMs devices can beformed on a surface of a first substrate (such as a silicon substrate).Bondline structures can be formed (e.g., positioned) on the firstsubstrate or a second substrate. The bondline structures can: define adistance that separates the first and second substrates; structurallybond the first substrate to the second substrate to form a unifiedsubstrate assembly; and hermetically seal a cavity enclosed by the firstand second substrates and the bondline structures.

Various wafer-to-wafer bonding processes for forming a hermeticallysealed cavity can include substances or conditions that can compromisedelicate components formed within the hermetically sealed cavity and/orextending under a bondline structure of the wafer-to-wafer bond. Forexample, high temperatures for melting eutectic substances and/or fusingglass frit can melt or accelerate chemical processes that degradeperformance of the delicate components. Similarly, the relatively hightemperatures can more quickly degrade lubrication systems in the cavityby heat-accelerated reactions of eutectic metallurgical substances(including, for example, selenium, indium and/or other low-temperaturematerials) with lubrication substances. Further, the lubrication systemsand anodic bonding used to form surface-fabricated MEMS structures cancontaminate otherwise clean and flat surfaces used to formwafer-to-wafer bonds. Also, pressures encountered in forming thewafer-to-wafer bonds cause thermocompression, which tends to damage CMOS(complementary-metal-oxide semiconductor) circuitry (including gates andrelated metallization).

In described examples, a MEMS device and/or a CMOS device is sealed insuch a cavity, such that the sealed device is environmentally protectedfrom an outside environment. Electrical signals can be coupled to andfrom the sealed device via electrical conductors traversing ahermetically sealed sidewall, for example, without compromising thecavity seal.

As described hereinbelow, a bonding structure is formed on a substrateto impede (and/or otherwise restrict) reactant species against migratinginto a cavity surrounded by the bonding structure. For example, thebondline structure can be arranged around (e.g., outwards from) thecavity, such that the migration of reactant species is impeded (e.g.,prevented) from against entering a headspace of the cavity. The bondingsubstances can include inert (or relatively inert) metals (e.g., goldand nickel), such that reactive substances (such as indium, seleniumand/or other reactant species) need not be intentionally included.Accordingly, outgassing from bonding substances in the sidewall of thebonding structure is minimized, such that contamination of sensitivestructures such as micromirrors (as well as the coating of lubricantand/or passivating agents thereof) is reduced.

As described hereinbelow, reliability and performance of a sealed devicecan be improved by processes and structures for sealing devices incavity formed during wafer-to-wafer bonding. The described processes andsidewall structures expose inert metals to the cavity, and are appliedat low temperatures and low bonding pressures. The low temperatures andlow bonding pressures used to form the sidewall structures helps protectmetallization and/or circuitry formed beneath (or above) the sidewall.

In an example, a plating process forms an gold overplated edge. The goldoverplated edge can be an overhanging portion of a gold cantileveredstructure that is cantilevered subsequent to the plating of the goldlayer by partially etching away an underlying resist. The goldoverplated edge includes a retrograde profile (e.g., as shown in FIG. 4,where a void exists underneath a suspended portion of the overplatedgold). Such gold overplated edges are formed on first and secondsubstrates, such that the first and second substrates can be bondedtogether (e.g., vertically bonded) by compressive forces. Thecompressive forces form a thermocompressive bond as a first goldoverplated edge is compressed against a second gold overplated edge(e.g., which includes a retrograde profile that is inverted and mirroredwith respect to the retrograde profile of the first gold overplatededge.

The thermocompressive bond can be formed at greatly reduced temperaturesand pressure (e.g., as compare against processes involving fusing and/ormelting of various eutectic substances). The thermocompressive bond canbe formed at room temperature by applying normal (e.g., orthogonal)vertical compressive forces. The compressive forces induce localizedvertical shearing of the first (e.g., lower) and second (e.g., upper)gold overplated edges, such that heat is locally generated by thevertical shearing. The vertical shearing welds the first and second goldoverplated edges together to form a hermetic seal around a cavity forincluding a sealed device. The welding can occur at low pressures (e.g.,atmospheric pressures) because the overplated structure deforms the goldedge in a localized area (e.g., which reduces net forces and pressure onthe substrate and/or intervening structures that would be otherwiseapplied). In various examples described below (e.g., with respect toFIG. 7, FIG. 8 and FIG. 9), the strength of the wafer-to-waferthermocompressive bonds can be increased by forming multiple concentricrings of shearing-induced sealing welds.

FIG. 1 is a cross-sectional diagram of an example first-substrateassembly for hermetic vertical shear weld wafer bonding. The assembly100 includes a first substrate 110: the first substrate 110 can be asemiconductor wafer (or die) formed from a crystalline lattice ofsilicon or gallium arsenide, for example. As shown in FIG. 5, a secondsubstrate 110 b can be used to form structures similar to and suitablefor bonding to the structures formed on the first substrate 110. Thesecond substrate 110 b can be of the same material as the firstsubstrate 100 a, or of a different material (e.g., glass, fortransmitting light to the sealed device).

The first substrate 110 (and the second substrate 110 b) can be formedin accordance with wafer-level processing to achieve an economy of scalein manufacture. (In other examples, die-level bonding processes andstructures can replace the wafer-level bonding processes and structuresdescribed herein.) The first substrate 110 can be formed with terminals(e.g., pins, not shown) on a lower or upper surface of the firstsubstrate 110 to electrically intercouple with other system devicesarranged outside of a sealed cavity to be formed on the first substrate110.

A seed layer 120 is deposited on the upper surface of the firstsubstrate 110. The seed layer 120 can be deposited by a chemical vapordeposition process and includes a deposited material suitable forforming a layer of a first metal thereupon. In an example, the firstmetal layer can be relatively “hard” metal (such as nickel, which is“hard” relative to the hardness of gold).

A resist structure 130 is formed over the seed layer. The resiststructure 130 delimits an edge for limiting a horizontal exent of thefirst metal to be deposited on the seed layer as described hereinbelowwith reference to FIG. 2.

FIG. 2 is a cross-sectional diagram of an example first-substrateassembly including a first metal for hermetic vertical shear weld waferbonding. Assembly 200 shows a first metal layer 240 (e.g., of nickel)deposited on the upper surface of the substrate 100. The resist 130includes a vertical surface adjacent to the first metal layer 240, whichdetermines the shape and location of the adjacent vertical surface ofthe first metal layer 240. The intersection of the first metal layer 240upper surface and the first metal layer 240 adjacent vertical surface isa fulcrum against which a hermetic vertical shear weld wafer bond can beformed (e.g., as described hereinbelow with respect to FIG. 6).

The first metal layer 240 can be deposited to a depth determined in partby the height of the resist 130, such that a flat surface is formed bythe upper surfaces of the resist 130 and the first metal layer 240. Theupper surfaces of the resist 130 and the hard metal layer 240 canoptionally be planarized to form the flat surface. The flat surface canbe used to deposit a layer of a second metal thereupon as describedhereinbelow with reference to FIG. 3.

FIG. 3 is a cross-sectional diagram of an example first-substrateassembly including a second metal for hermetic vertical shear weld waferbonding. Assembly 300 includes a second metal layer 350 deposited overthe upper surfaces of the resist 130 and the first metal layer 240.

In an example, the second metal layer 350 is a “soft” metal (e.g., gold)relative to the hardness of the metal (e.g., nickel) of the first metallayer 240. For example, the first metal layer 240 retains its shape(e.g., because of the relative hardness to the second metal layer 350),including the shape (e.g., edge) of the fulcrum around which the secondmetal is deformed (e.g., bent and sheared). The second metal layer 350is deformed around the fulcrum by compressive forces applied for forminga hermetic vertical shear weld wafer bond (e.g., as describedhereinbelow with respect to FIG. 6).

The second metal layer 350 can be patterned to cover the first metallayer 240, the resist 130 and the vertical interface between (e.g.,adjacent to both) the first metal layer 240 and the resist 130. Thesecond metal layer 350 includes a chamfered edge (e.g., a radiused edge,as shown in profile in FIG. 3), which is formed (e.g., formed at leastin part) over the resist 130. The chamfered edge includes a slopedprofile for “self-centering” a second-substrate assembly during awafer-to-wafer bonding of the first- and second-substrates as describedhereinbelow with respect to FIG. 5. The radius of the chamfered edgeprovides a net horizontal component of force when the second-substrateassembly is misaligned (e.g., slightly misaligned), such that he nethorizontal component of force tends to correct (e.g., tends toself-center) for the horizontal misalignment of the first- andsecond-substrate assemblies by urging the first- and second-substrateassemblies into horizontal alignment.

As described herein below with reference to FIG. 4, the resist 130(e.g., which is below and subjacent to the chamfered edge of the secondmetal layer 350) is evacuated to expose a vertical edge of the fulcrumof the first metal layer 240.

FIG. 4 is a cross-sectional diagram of an example first-substrateassembly including an exposed vertical edge of a fulcrum for hermeticvertical shear weld wafer bonding. Assembly 400 includes an exposedvertical edge 452 of a fulcrum. The exposed vertical edge of a fulcrumis exposed by evacuating (e.g., etching away) the resist 130.

Accordingly, the chamfered edge of the second metal layer 350 overhangs(e.g., is cantilevered) over the substrate 110, such that a void existsbeneath the cantilevered portion of the second metal layer 350. The voidbeneath the cantilevered portion of the second metal layer 350 includesa space into which the second metal layer 350 can be bent and deformedas described hereinbelow.

FIG. 5 is a cross-sectional diagram of example first- andsecond-substrate assemblies for hermetic vertical shear weld waferbonding. Assembly 500 includes assemblies 400 a and 400 b. Assembly 400a is an assembly such as assembly 400, whereas assembly 400 b is anassembly similar to the assembly 400. The assembly 400 b includessidewall structures (such as a first metal layer 240 b and a secondmetal layer 350 b) that are positioned (or otherwise offset) on thesecond substrate to be aligned with respective edges of the firstassembly 400 a. The alignment intersperses prominences of the assemblies400 a and 400 b for vertical mating, For example, the respectivechamfered edges (of the second metal layers 350 a and 350 b) aremutually sheared when vertically compressed together (as describedhereinbelow with respect to FIG. 6 and FIG. 7).

The assembly 400 a includes a substrate 110 a, first metal layer 240 a,second metal layer 350 a and vertical edge 452 a of a lower fulcrum(which respectively correspond to the substrate 110, first metal layer240, and second metal layer 350 and vertical edge 452 of the firstsubstrate 110). Similarly, the assembly 400 b includes a substrate 110b, first metal layer 240 b, second metal layer 350 b and vertical edge452 a of an upper fulcrum (which respectively correspond to thesubstrate 110, first metal layer 240, and second metal layer 350 andvertical edge 452 of the first substrate 110). The structures of theassembly 400 b are inverted and mirrored with respect to thecorresponding structures of the assembly 400 a.

As described hereinbelow with respect to FIG. 6, the vertical edge 452 aof the upper fulcrum and the vertical edge 452 b of the lower fulcrumare vertically aligned and offset, such that the chamfered edges of thesecond metal structures 350 a and 350 b are deformed around the edges ofthe respective fulcrums of the first metal structures 240 a and 240 b inresponse to forces generated while vertically compressing the firstassembly 400 a and the second assembly 400 b together.

FIG. 6 is a cross-sectional diagram of example joined first- andsecond-substrate assemblies for hermetic vertical shear weld waferbonding. Assembly 600 includes the assemblies 400 a and 400 b (describedhereinabove with respect to FIG. 4 and FIG. 5), wherein the assemblies400 a and 400 b are bonded (e.g., fused) together and hermeticallysealed by the weld 660.

The weld 660 is formed in response to forces generated while compressingthe first assembly 400 a and the second assembly 400 b together. Forexample, the assembly 400 a can be formed on a first wafer (such asdescribed hereinbelow with respect of FIG. 9) and the assembly 400 b canbe formed on a second wafer (such as described hereinbelow with respectof FIG. 9), which are compressed together during wafer-level processing(e.g., before singulation for producing individual dies).

As the first assembly 400 a and the second assembly 400 b are compressedtogether, the chamfered edges of the second metal layers 350 a and 350 bcome into contact. The second metal layers 350 a and 350 b can includean inert, ductile metal such as gold. The area of contact betweencontacting portions of the second metal layers 350 a and 350 b isrelatively small, which increases the localized pressures to valuessubstantially higher than pressures mutually exerted between each of thefirst metal layers 240 a and 240 b and their respective substrates 110 aand 110 b (as well as any intervening components or structures betweenany adjacent layers and substrates). Accordingly, relatively high forcesare applied to the chamfered edges of the second metal layers 350 a and350 b, which are sufficiently high to deform (and optionally meltportions of) the chamfered edges without damaging the first metallayers, the respective substrates and/or any intervening components.

As the chamfered edges of the second metal layers 350 a and 350 b comeinto contact, torque is applied to each cantilevered portion of thesecond metal layers 350 a and 350 b. The torque is applied with respectto (e.g., around) a fulcrum formed by the arrangement (e.g.,intersection) of the exposed vertical and adjacent (e.g., contacting arespective second metal layer 350 a or 350 b) horizontal faces of arespective first metal layers 240 a or 240 b. Accordingly, each fulcrum(which is formed by a first metal layer 240 a or 240 b) contacts arespective cantilevered portion of a second metal layer 350 a or 350 b.

As the first assembly 400 a and the second assembly 400 b continue to befurther compressed together, each cantilevered portion of the secondmetal layers 350 a and 350 b is deformed: the cantilevered portion ofthe second metal layer 350 a is bent in a generally downwards direction,whereas the cantilevered portion of the second metal layer 350 b is bentin a generally upwards direction. The bending (e.g., which includescompressive, tensile and shear forces) of each such cantileveredportion—and friction (e.g., the friction opposing the slippage acrossthe contacting surfaces of the second metal layers 350 a and 350b)—generates localized heat sufficient to melt the interface betweensecond metal layers 350 a and 350 b, such that a weld 660 (e.g., ahermetic vertical shear weld wafer bond) can be formed by fusingcontacting portions of the second metal layers 350 a and 350 b.

After such welding, the hermetic vertical shear weld wafer bond formedby the weld 660 generates forces for bonding (e.g., fixedly bonding) thefirst assembly 400 a and the second assembly 400 b together as well asimpedes the migration of contaminants such as reactant species acrossthe weld 660 into a cavity, described hereinbelow with respect to FIG.7.

FIG. 7 is a cross-sectional elevational view of an example two-substrateassembly that includes example hermetic vertical shear weld wafer bonds.For example, the two-substrate assembly 700 (shown in cross-section)includes a first (e.g., lower) substrate 400 a and a second (e.g.,upper) substrate 400 b. The first substrate 400 a includes multipleinstances of the first metal layers 740 a (shown in cross-section),which are arranged (e.g., as rings 820, 840 and 860, as shown in FIG. 8and FIG. 9) around a perimeter of the cavity 770 (e.g., which is to behermetically sealed). The second substrate assembly 400 b includesmultiple instances of the first metal layers 740 b (shown incross-section), which are arranged around a perimeter of the cavity 770and are positioned (e.g., interdigitated) between instances of the firstmetal layers 740 a, such that multiple instances of a shear weld isformed (e.g., by compressing the substrate assemblies 400 a and 400 btogether).

A shear weld is formed in the void between each first (e.g., lower)substrate 400 a first (e.g., hard) metal layer 740 a and a second (e.g.,upper) substrate 400 b first (e.g., hard) metal layer 740 a. Forexample, assemblies 600 as shown in FIG. 7 include a shear weld asdescribed hereinabove with respect to FIG. 6. The shear welds joinadjacent segments of the second (e.g., soft) metal layers 740 a and 740b (e.g., formed as separate segments on each of the first substrate 400a and the second substrate 400 b) into a unified (e.g., continuous) seal750.

The seal 750 extends outwards from the cavity 770 (e.g., to a saw lane,not shown) and extends around the perimeter the cavity (e.g., as shownin top view in FIG. 8 and FIG. 9), such that the cavity 770 is ahermetically sealed environment in which the sealed device 780 isprotected from reactant species. The multiple instances of the shearweld, which ring the cavity 770 on all four sides, helps to increase theimpermeabilty of the seal formed by the seal 750. Further, the seal 750is an inert material, such that the sealed device 780 is not exposed toreactive species from bonding agents otherwise present in a sidewallstructure.

Accordingly, the first and second metal layers form sidewalls forbonding the first substrate 400 a and the second substrate 400 b and forsealing the cavity 770. The sidewalls are positioned to protectsensitive components 180 of a chip (e.g., singulated die) within cavityperipherally supported by the sidewalls. The included device 780 caninclude an array of micromirrors (not shown) coupled to the firstsubstrate 400 a, where the performance of each micromirror couldotherwise be degraded by the presence of reactive species or moisturefrom bonding agents or operational environments. Accordingly, the firstsubstrate 400 a, the second substrate 400 b and the seal 700 helpprevent the intrusion of contaminants such as reactant species, gassesand/or moisture into the cavity 770.

FIG. 8 is a top-view diagram of an example first-substrate assembly forhermetic vertical shear weld wafer bonding. The assembly 800 includes afirst substrate 810 that includes an outer ring 820, and intermediatering 840 and an inner ring 860 arranged for bonding to similarstructures on a second substrate (not shown).

For example, the outer ring 820, and intermediate ring 840 and an innerring 860 are metal layers, such as the first (e.g., hard) metal layers(e.g., 740 a and 340 a described hereinabove). As described hereinabove(e.g., with reference to FIG. 7), each of the rings extend upwards fromthe substrate 810, such that an outer valley 830 is formed between theouter ring 820 and the intermediate ring 840, and such that an innervalley 850 is formed between the intermediate ring 840 and the innerring 860.

The rings of the second substrate (not shown) are arranged to mate withthe rings 820, 840 and 860, such that the shear welds (e.g., shear welds660) are formed in the voids adjacent (e.g., closely adjacent) to theouter ring 820, the intermediate ring 840 and the inner ring 860.Accordingly, each of the outer valley 830 and the inner valley 850 arearranged for including two shear welds and a second (e.g., soft) metallayer initially formed over corresponding first (e.g., hard) metallayers of the second substrate. (FIG. 7 shows an example configurationof hermetic vertical shear weld wafer bonding of a first substrate andthe second substrate in cross-section.)

In response to the formation of the vertical shear welds, the cavity 770is hermetically sealed, which protects the sealed device 780 againstmigration of reactant species, environmental gasses and moisture. Themultiple rings enhance the degree of impermeability of the seal andincrease the strength of the bonding forces between the upper and lowersubstrates.

FIG. 9 is a top-view diagram of an example first-substrate wafer forhermetic vertical shear weld wafer bonding. The wafer 900 includes asubstrate 910 (which is a substrate such as the substrate 810, describedhereinabove). Multiple instances of the assembly 800 are formed on thesubstrate 910. A corresponding wafer (e.g., arranged-to-fit wafer, notshown) includes similar structures positioned to mate within the valleysformed by the rings of each of the instances of the assembly 800.Accordingly, the wafer 900 (and the structures formed thereon) isarranged to mate with a corresponding wafer (not shown) in response tocompressive forces applied to join and bond the wafer 900 and thecorresponding wafer together.

FIG. 10 is a cross-sectional diagram of dimensioning of components ofexample first- and second-substrate assemblies for hermetic verticalshear weld wafer bonding. For example, relative sizing and spacing ofcomponents in diagram 1000 can be described by the variables A, B and C:A is the depth of the first (e.g., nickel) metal layer 240 a (and 240b); B is the depth of the second (e.g., gold) metal layer 350 a (and 350b); and C is the width between the vertical projections of the exposedvertical edge 452 a (of fulcrum 1042 a) and the exposed vertical edge452 b (of fulcrum 1042 b).

An example minimum spacing C for forming a complete vertical shear weldis:

$\begin{matrix}{C_{\min} \geq \frac{\pi \; B^{2}}{2\left( {A + B} \right)}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

An example maximum spacing C for forming a complete vertical shear weldis:

C _(max)≤2B  (Eq. 2)

An example optimum spacing C for forming a complete vertical shear weldis:

C _(optimum)=√{square root over (2B)}  (Eq. 3)

In an example where A is 5 microns, and B is 5 microns: C_(max)≤10microns; C_(min)≥3.9 microns; and C_(optimum)=7.1 microns.

Accordingly, hermetic vertical shear weld wafer bonding can be formed inaccordance with wafer-level processing to achieve an economy of scale.The first metal layer 240 a can be formed over a conductor and/or CMOScircuitry 1010, such that net forces for forming the hermetic verticalshear weld wafer bonding are not directly applied (and instead aredistributed over greater areas), and such that the pressure applied tothe underlying conductor and/or CMOS circuitry 1010 is substantiallyreduced (e.g., reduced sufficiently such that sufficient bondingpressure for generating vertical shear welds can be applied withoutdamaging the underlying conductor and/or CMOS circuitry 1010).

FIG. 11 is another cross-sectional diagram of example first- andsecond-substrate assemblies for hermetic vertical shear weld waferbonding. The diagram 1100 includes a lower element 1160 having anexposed vertical edge 1162 and a cantilever 1166. The diagram 1100includes an upper element 1164.

A method of forming an apparatus has been introduced. The methodincludes depositing a resist on a first substrate peripherally around acavity including a first surface defiled by the first substrate. A firstmetal layer is deposited over the first substrate, wherein the firstmetal layer is deposited against a first vertical edge of the resist. Asecond metal layer is deposited over the first metal layer and theresist. The resist is removed to form a second metal layer cantileverand a void that extends between the second metal layer cantilever andthe first substrate. A second substrate is bonded to the first substratein response to a contacting structure of the second substrate deformingthe second metal layer cantilever, wherein the contacting structure ofthe second substrate is forced against the second metal layer cantileverin response to compressing the first and second substrates together.

Modifications are possible in the described examples, and other examplesare possible, within the scope of the claims.

What is claimed is:
 1. Apparatus, comprising: a first substrate thatdelimits a first surface of a cavity; a second substrate that delimits asecond surface of the cavity; a first metal layer along a periphery ofthe cavity, the first metal layer including a first horizontal surfaceformed adjacent the first substrate, and the first metal layer includingan exposed vertical edge; a second metal layer adjacent a secondhorizontal surface of the first metal layer, the second metal layerincluding a cantilevered portion that extends beyond the exposedvertical edge, such that a void is between the cantilevered portion andthe first substrate; and a fulcrum at an intersection between the secondhorizontal surface of the first metal layer and the exposed verticaledge of the first metal layer; in which the cantilevered portion of thesecond metal layer is configured to deform around the fulcrum and intothe void by bonding the first substrate to the second substrate.
 2. Theapparatus of claim 1, wherein the second metal layer is configured toimpede a contaminant against migrating across the first metal layer andentering the cavity after bonding to the second substrate.
 3. Theapparatus of claim 1, wherein the second metal layer includes a metalmore ductile that the first metal layer.
 4. The apparatus of claim 1,wherein the second metal layer includes gold, and the first metal layerincludes nickel.
 5. The apparatus of claim 1, wherein the secondsubstrate includes a metal contacting structure configured to contactthe cantilevered portion of the second metal layer.
 6. The apparatus ofclaim 5, wherein the cantilevered portion of the second metal layerforms a shear weld with the metal contacting structure by bonding thefirst substrate to the second substrate.
 7. The apparatus of claim 1,wherein the exposed vertical edge is a first exposed vertical edge, thecantilevered portion is a first cantilevered portion, the void is afirst void, the fulcrum is a first fulcrum, and the apparatus includes:a third metal layer including a third horizontal surface formed adjacentthe second substrate, the third metal layer including a second exposedvertical edge; a fourth metal layer adjacent a fourth horizontal surfaceof the third metal layer, the fourth metal layer including a secondcantilevered portion that extends beyond the second exposed verticaledge, such that a second void is between the second cantilevered portionand the second substrate; a second fulcrum at an intersection betweenthe fourth horizontal surface of the third metal layer and the secondexposed vertical edge; in which the second cantilevered portion of thefourth metal layer is configured to deform around the second fulcrum andinto the second void by bonding the first substrate to the secondsubstrate.
 8. The apparatus of claim 7, wherein a shear weld is formedbetween the first cantilevered portion and the second cantileveredportion.
 9. The apparatus of claim 8, wherein the first metal layerincludes a third exposed vertical edge, the second metal layer includesa third cantilevered portion that extends beyond the third exposedvertical edge, such that a third void is between the third cantileveredportion and the first substrate, a third fulcrum is located at anintersection between the second horizontal surface of the first metallayer and the third exposed vertical edge, and the third cantileveredportion is configured to deform around the third fulcrum and into thethird void by bonding the first substrate to the second substrate. 10.The apparatus of claim 1, wherein the first substrate includes silicon,and the second substrate includes glass.
 11. The apparatus of claim 1,wherein the exposed vertical edge is a first exposed vertical edge, thecantilevered portion is a first cantilevered portion, the void is afirst void, the fulcrum is a first fulcrum, the first metal layerincludes a second exposed vertical edge, the second metal layer includesa second cantilevered portion that extends beyond the second exposedvertical edge, such that a second void is between the secondcantilevered portion and the first substrate, a second fulcrum islocated at an intersection between the second horizontal surface of thefirst metal layer and the second exposed vertical edge, and the secondcantilevered portion is configured to deform around the second fulcrumand into the second void by bonding the first substrate to the secondsubstrate.
 12. The apparatus of claim 1, further comprising circuitryextending between the first substrate and the first metal layer. 13.Apparatus, comprising: a first substrate that delimits a first surfaceof a cavity; a second substrate that delimits a second surface of thecavity; a first metal layer along a periphery of the cavity, the firstmetal layer including a first horizontal surface formed adjacent thefirst substrate, and the first metal layer including an exposed verticaledge; and a second metal layer adjacent a second horizontal surface ofthe first metal layer, the second metal layer including a cantileveredportion that extends beyond the exposed vertical edge, such that a voidis between the cantilevered portion and the first substrate; in whichthe cantilevered portion of the second metal layer is configured to weldto a contacting structure of the second substrate by bonding the firstsubstrate to the second substrate, and the welded cantilevered portionof the second metal layer is configured to impede contaminants againstcontacting an element within the cavity.
 14. The apparatus of claim 13,wherein the element is a microelectromechanical system (MEMS) element.15. The apparatus of claim 13, wherein the weld extends peripherallyaround the cavity.
 16. The apparatus of claim 13, wherein the weld isconfigured to exert tensile forces for bonding the first substrate tothe second substrate.